1. Field of the Invention
This invention relates to microstrip patch antenna arrays having applications in the fields of communications and radar. Microstrip patch antennas are particularly useful for spacecraft and aircraft applications on account of their light weight and flat profile.
2. Discussion of Prior Art
A section of a conventional microstripline is shown in FIG. 1. It comprises a conducting ground plane 1, a dielectric spacer 2 and a conductor 3. For a straight, infinitely long strip, virtually no radiation will occur as long as the separation between the conductor 3 and ground plane 1 is small compared with the wavelength of the propagating wave. However, in the presence of a discontinuity, the field in the gap between the conductor 3 and the ground plane 1 becomes unbalanced and the gap radiates.
Any patch of microstrip such as the patch 4 shown in FIG. 2 has a radiating aperture around its rim. If fields and currents are excited by a stripline feed 5, for example, the patch 4 will radiate. The shape of the patch and method and location of its feed determine the field distribution and therefore its radiation characteristics. The most commonly used patches are rectangular, square or circular, such patches producing a fairly broad, single beam of radiation in a direction normal to their surfaces and in the case of rectangular patches, producing a controllable polarisation effect.
Microstrip patches are most commonly used in planar arrays for applications where a narrow beam pattern is required. A plan-view of a typical planar microstrip patch array layout is shown in FIG. 3. It comprises a plurality of rectangular conducting patches 6 fed via a microstrip feedline 7 which is printed onto the same substrate as the patches. The array shown in FIG. 3 has a narrow single beam pattern.
Other discontinuities such as apertures in an otherwise uniform conducting layer will also cause the generation of radiation in the same way, and the term "patch" as used hereinafter shall include such apertures.
It is an object of the present invention to provide a microstrip patch array having a multiple beam capability to facilitate simultaneous or switched coverage of a wide field of view.
Hitherto, multiple beam arrays have been formed by feeding appropriately grouped radiating elements (microstrip patches, for example) via a "beamforming" circuit. A well-known example of a beamforming circuit is the so-called Blass matrix which is shown schematically in FIG. 4. It comprises a grid of transmission lines and directional couplers 8 which couple input power applied to beam ports 9 and 10 to radiating patches 11 (12a to 12f are matched loads). Patch spacing and interconnecting line lengths determine beam direction. In the arrangement of FIG. 4, the number of beams is equal to the number of beam ports.